An Active Matrix Organic Light Emitting Diode (AMOLED) display is one of hotspots in a field of flat display research currently. As compared with a liquid crystal display, the Organic Light Emitting Diode (OLED) has advantages of a low power consumption, a low cost of production, a self-luminescent feature, a wide angle of view, a rapid response speed and so on, and has begun to replace a traditional LCD display screen in the display fields of a mobile phone, a PDA (Personal Digital Assistant), a digital photo frame, etc. A design for a pixel driving circuit is a core technique for the AMOLED display and has important research significance.
Unlike a TFT-LCD (Thin Film Transistor Liquid Crystal Display) which controls brightness by a stable voltage, the OLED is driven by a current and requires a stable current to control its light-emitting. In an existing driving circuit having two transistor T1, T2 and one storage capacitor C1(as illustrated in FIG. 1), a driving current IOLED is a current generated by a voltage Vdata, provided from a data line, acting on a saturation region of a driving transistor (DTFT). This current drives the OLED to emit light, wherein a calculation formula of the driving current is IOLED=K(VGS−Vth)2, where VGS is a voltage between a gate and a source of the driving transistor, Vth is a threshold voltage of the driving transistor. For the reasons of the technology processes, device degradations and the like, there is an unhomogeneity among the threshold voltages (Vth) of the driving TFTs in the respective pixels. The unhomogeneity existing among the threshold voltages of the driving TFTs (for example, T2 in FIG. 1) in the respective pixels results in changes in the currents flowing through each pixel OLED, and in turn affects a display effect of an entire image.